The present invention is in the field of photometric circuits for cameras and the like.
It is well known in the art to provide photometric circuits in cameras or the like for determining the required film exposure time depending upon the conditions of film speed, lens aperature setting and scene brightness. The determination is made in a circuit which electronically calculates the Apex indication value Tv according to the Apex indication formula: EQU S.sub.v +B.sub.v =T.sub.v +A.sub.v,
where S.sub.v, B.sub.v, and A.sub.v are the Apex indication values for film sensitivity, scene brightness, and lens aperature opening, respectively.
The conventional photometric circuit utilizes a plurality of operational amplifiers and precision wide range variable resistors. This causes several drawbacks which will be explained in greater detail below. An example of a conventional photometric circuit is shown in FIG. 1.
In the circuit of FIG. 1, a variable resistor 1, which can be set in accordance with the film sensitivity, and diodes 2 and 3 are connected in series with an electric power source 14. The junction of variable resistor 1 and diode 2 is connected to the input of an operational amplifier 4 constituting a voltage-follower circuit. Another variable resistor 11, which can be set in accordance with the aperture setting, and diodes 12 and 13 are also connected in series with the electric power source 14. The junction of variable resistor 11 and diode 12 is connected to an input of another operational amplifier 10 constituting a voltage-follower circuit. A photoelectric element 5, adapted to receive light from the scene, is connected to the collector of a transistor 6 having an emitter connected with the output of the operational amplifier 4. A high input resistance operational amplifier 7, constituting a voltage-follower circuit, is connected between the collector and base of transistor 6, thereby forming a feedback circuit. An integrating capacitor 8 is connected to the collector of another transistor 9 having a base connected to the base of the transistor 6. The emitter of transistor 6 is connected with the output of the operational amplifier 10.
The operation of the above described circuit is well known. The values of the currents and voltages are such that the voltages V.sub.B, V.sub.A, and V.sub.D correspond directly to the apex-indicative values B.sub.v, S.sub.v, and A.sub.v of the brightness B of the object, the sensitivity S of the film, and the aperture A of the lens, respectively. Therefore V.sub.C, which satisfies the circuit equation: EQU V.sub.C =V.sub.B +(V.sub.A -V.sub.D) (1)
is equal to a voltage corresponding to B.sub.v +S.sub.v -A.sub.v, that is, corresponding to an apex-indicative value T.sub.v of the exposure T. Because of the base-emitter voltage versus collector current characteristic of transistor 9, the collector current of the transistor is inversely proportional to the exposure time T. When a timing switch 15 of the camera is opened, the integrating capacitor 8 begins to charge and current flows through a coil 17 of an electromagnet. When the voltage across capacitor 8 exceeds the threshold voltage of a switching circuit 16, the energization of the electromagnet coil 17 is terminated. The time period starting from the opening of the timing switch 15 to the termination of the energization of the coil 17 is thus proportional to the value T calculated as described above, and constitutes the exposure period of the camera.
However, the aforesaid photometric circuit has drawbacks as follows.
(1) The composition of the operational amplifiers is complicated and the cost of the circuit is therefor high.
(2) Because of the threshold voltages of the operational amplifiers and the transistors, a relatively high voltage power source must be used.
(3) Because of the operational amplifiers and the variable resistors 1 and 11, the current consumption of the circuit is high.
(4) Because of the wide varying ranges of the variable resistors, corresponding to the aperture of the lens and the sensitivity of the film, the production of the variable resistors is difficult.